Tx ID as Asset ID (#926)

* Allow AssetLinks to be used in place of Assets in the Transaction Model and enforce `Transaction.transfer()` to only take an AssetLink

* Remove AssetLink's inheritance from Asset

* Remove id from the Asset model

* Fix get_txids_by_asset_id query for rethinkdb after removing asset's uuid

Because `CREATE` transactions don't have an asset that contains an id
anymore, one way to find all the transactions related to an asset is to
query the database twice: once for the `CREATE` transaction and another
for the `TRANSFER` transactions.

* Add TODO notice for vote test utils to be fixtures

* Update asset model documentation to reflect usage of transaction id

* Fix outdated asset description in transaction schema

docs/server/generate_http_server_api_documentation.py

@@ -62,7 +62,7 @@ def main():
     """ Main function """
     privkey = 'CfdqtD7sS7FgkMoGPXw55MVGGFwQLAoHYTcBhZDtF99Z'
     pubkey = '4K9sWUMFwTgaDGPfdynrbxWqWS6sWmKbZoTjxLtVUibD'
-    asset = Asset(None, 'e6969f87-4fc9-4467-b62a-f0dfa1c85002')
+    asset = Asset(None)
     tx = Transaction.create([pubkey], [([pubkey], 1)], asset=asset)
     tx = tx.sign([privkey])
     tx_json = json.dumps(tx.to_dict(), indent=2, sort_keys=True)